This invention relates to SO.sub.2 flue gas scrubbing systems using calcium compounds as an additive and in particular to a method of improving additive utilization and reducing the required solid content of the circulating slurry.
Calcium compounds such as calcium carbonate and calcium hydroxide are frequently used in SO.sub.2 flue gas scrubbing systems. In such systems, the calcium compound is dissolved in water and recirculated in contact with the flue gas. The SO.sub.2 combines with the calcium compound to form calcium sulfite, and a portion of this is oxidized to calcium sulfate. While the calcium compound is preferably completely dissolved in the water prior to adding the solution to the recirculating scrubber system, this is not practically possible since the calcium compounds are difficult to dissolve. Accordingly, some undissolved compound remains in the slurry, and it will dissolve in time if it is left in the system. Only after it is dissolved is it effective in performing the scrubbing operation. Any of the calcium compound which is sent to waste before dissolving is wasted.
Calcium sulfate forming in the system has a tendency to deposit and build up on the internal scrubber surfaces. It has been found that this incrustration can be avoided by maintaining sufficient calcium sulfate crystals in the slurry as a seed. The additional calcium sulfate forming will preferentially deposit on the circulating crystals rather than on the scrubber surfaces.
In order to remove the sulfur compounds from the system, minimize wear on the components, and maintain the slurry in a reasonably fluid condition, a slip stream of the recirculated slurry is withdrawn and passed to waste. The liquid portion is normally returned to the slurry system.
Normal operation of the system requires the circulation of high total solids concentrations in order to provide ample calcium sulfate seed crystals to prevent scaling. One method used to minimize the total solids concentrations is forced oxidation where air is pumped through the slurry mixture. This oxidizes a portion of the sulfite crystals to sulfate, thereby providing a sufficient amount of seed crystals. Such operation, however, is expensive since it requires additional investment and operating power to pump air through the system.